1. Field of the Invention
The present invention relates to a manual air pump for inflating, e.g., bicycle tires, and more particularly to dual-mode manual air pump that may in one mode provide a very high volume of air per stroke and in another mode provide less volume of air at high pressure.
2. Description of the Related Art
Bicycle pumps include two typical types: 1) big bore for high volume per stroke and low pressure for rapid inflation, yet high pressure operation is difficult to achieve; and 2) small bore for low volume per stroke and high pressure for inflating bicycle tires to high pressure. U.S. Pat. No. 5,443,370 issued to Wang on Aug. 22, 1995 proposes a two-cylinder manual air pump having selectable high volume and high pressure modes, wherein the pump in one mode provides a very high volume of air per pump stroke and in another mode provides less volume per stroke but enables the user to inflate to higher pressures. Nevertheless, manufacture, assembly, and use of such manual air pump are complicated and troublesome. The structure in the cylinders for air passage is too complicated. In addition, the thinner piston rod might be broken during the output stroke, as the travel for the thinner piston rod is relatively long and the piston on the thinner piston rod encounters much higher pressure when the piston approaches the inner end of the outer cylinder. Namely, such a thinner piston rod tends to be broken and requires a larger force to push it.